This invention relates to a frequency synthesizer and, more particularly, to a synthesizer employing a phase locked loop having a mixer outputting a comb frequency spectrum for down conversion of an outputted frequency, and including circuitry for providing a fractional comb frequency offset of the outputted frequency to avoid aliasing of mixer output signals.
Synthesizers of high frequencies, in the gigahertz range by way of example, frequently employ a phase locked loop to stabilize the output signal frequency, and include also circuitry for down conversion in frequency of a sample of the output signal to enable use of lower frequency circuit components, such as those operating in the megahertz range, for phase locking the output signal to an input reference signal. Generally, it has been the practice to employ a frequency divider comprising one or more digital counters to accomplish the down conversion of frequency.
In the operation of the frequency divider, the counters divide both the frequency and the phase of the sample of the output signal. This operation is satisfactory for many applications, but suffers from a limitation in that there may be excessive phase noise in the output signal for applications requiring a precision output signal. For example, in the case of a divider which divides the frequency and the phase by a factor of ten for presentation to a phase detector of the phase locked loop, a phase deviation at the low frequency end of the divider is one tenth that at the high frequency end of the divider. Correspondingly, the phase noise power at the output signal is greater than the phase noise power at the input to the phase detector by a factor of 100 (20 decibels).
Thus, there is a problem in the use of a frequency divider in a phase locked loop in that there results a phase noise in the output signal which may be excessive for generation of precision frequency in the output signal. Conceptually, it is possible to provide the down conversion by mixing the sample of output signal with a reference signal which differs in frequency from the output signal by the desired value of intermediate frequency (IF) for operation of the phase detector. The mixing process avoids generation of the foregoing phase noise. The desired IF signal can be obtained by use of a mixer outputting a comb frequency spectrum wherein individual spectral lines are separated by harmonics of an input reference frequency. Such a mixer may be referred to as a sampling mixer in the frequency domain by analogy with the generation, in the time domain, of a series of sampling pulses by a pulse generator. However, heretofore, such mixers have not had general acceptance because the numerous output spectral lines introduce aliasing which inhibits extraction of the desired IF signal.